1. Field of the Invention
The invention relates to a display for motor vehicles, in which an image on the display having an upwardly facing display surface is guided toward the visual point of a driver through a reflecting member such as a windshield so that the image is visually recognized as a vertual image behind the reflecting member.
2. Background
A conventional display for motor vehicles is designed to allow a driver to identify a virtual image of a display image in front of a motor vehicle. That is, in order to remotely display an image as a virtual image in front of the motor vehicle, the display is arranged within a dashboard with the display surface thereof facing upwardly, and light emitted from the image on the display is reflected by the windshield through an opening arranged in the dashboard so that the light is guided toward the visual point of the driver.
However, since the display surface faces upwardly external light, such as sunlight, transmitted through the windshield tends to be transmitted onto the display surface which induces a "washout" phenomenon, making the display extremely hard to see. Referring to FIG. 7, to overcome this problem, a first prism 11 and a second prism 12 having the same vertical angles .theta. are arranged immediately above the display surface of a display 1 in such a manner that the vertical angles .theta. thereof oppose each other. The surfaces other than the two surfaces forming the vertical angle .theta. of each of the first and second prisms 11, 12 are coated with a dark color. Reference numeral 6 designates a bonnet for covering a combustion chamber.
In this case, as external light 3 enters the display 1 through windshield 4, the external light 3 is reflected by a surface 12a, at which the second prism 12 is placed over the first prism 11, in the direction of arrow A. As a result, the external light 3 is prevented from being transmitted onto the display surface of the display 1, thus leaving the display image free from washout. Further, external light is prevented from being transmitted through the sides of the first and second prisms 11, 12 due to the dark coating on these surfaces, thus further preventing the light from reaching the display 1.
Since the prisms 11, 12 having the same shape are overlapped one upon the other so that their vertical angles oppose each other, light exiting from the display 1 is little affected when transmitted through the prisms 11, 12. Thus, the display image is not distorted when seen from a visual point 5. This allows the display image to be visually identified as a virtual image at the back of a reflecting section 4a of the windshield 4 (in front of the motor vehicle).
Accordingly, if the prisms 11, 12 are placed immediately above the display surface of the display 1 as described above, reflection of the external light 3 can be regulated. However, an optical axis B extending toward the visual point 5 such as shown in FIG. 7 is established. That is, light emanating from the display is reflected by surface 12b of the second prism 12, transmitted through surface 12c, and reflected by the windshield 4 surface along the optical axis B. In this case, the angle of incidence of light transmitted onto the surface 12b from the display 1 is so wide that the reflectance of the light becomes large, which causes a highly luminous ghost-like image.
This state will be described in detail with reference to FIG. 8. The optical axis B is defined by a ray of light D and a ray of light E among a plurality of rays of light transmitted from the display 1. That is, the ray of light D and the ray of light E transmitted from a display range C establishes the optical axis B due to conditions including the inclination of the windshield 4, the distance between the display 1 and the prisms 11, 12, etc. The display range C is disposed toward the front of the motor vehicle on the display surface of the display 1. Therefore, when viewed from the visual point 5, undesired ghost-like images are seen below the desired display image, as shown in FIG. 9.